Miterm review

I do not know why clay rich in Montmorillonite will swell, can anyone explain this to me? Another question is that from the 2005 midterm questions, there is one questions asking me distinguish regolith and solum but I don't know these two words, do I need to know?

Soil rich in montmoriollinite type of phylosilicate minerals will swell when wet (and shrink when dry) since adjacent layers in the crystalline structure of this mineral are loosely attracted to each other. Consequently, when there is abundance of water in soil water molecules and hydrated ions can enter in the interlayer space of montmoriollinite and push adjacent layers further apart, causing swelling. Regolith and solum were defined during our 2nd lecture. Regolith refers to A, B and C horizons (i.e., it is the unconsolidated material above solid rock), while solum refers to A and B horizons only (i.e., it is the upper & most weathered part of the soil profile).